The present invention relates to an apparatus for controlling the amount of secondary air fed into an intake passage or into an exhaust passage of an internal combustion engine for controlling an equivalent air-fuel ratio within a predetermined range.
In the field of this art, a method is known in which the equivalent air-fuel ratio is detected by an air-fuel ratio sensor, for example, an oxygen concentration sensor for detecting the concentration of oxygen in the exhaust gas, and then secondary air is fed into an intake passage or into an exhaust passage of an internal combustion engine according to the detected equivalent air-fuel ratio, for maintaining the equivalent air-fuel ratio within a predetermined range which is near the stoichiometric air-fuel ratio, whereby the effect of purifying pollutants in a three-way catalytic converter disposed in the exhaust system is improved.
In a conventional apparatus for carrying out the above-mentioned method, the amount of secondary air to be injected into the engine is controlled by an air flow control valve disposed in a passage between an air pump and a secondary air injection mechanism. The air flow control valve is driven by an absolute pressure signal applied thereto through an electromagnetic valve which is adapted for switching the transmission of the absolute pressure on or off in response to an electrical signal provided from the air-fuel ratio sensor.
In order to effectively carry out air-fuel ratio control processes wherein, the amount of secondary air fed into the engine is controlled, the driving speed of the air flow control valve should be rapidly changed when the operation for increasing or decreasing the amount of secondary air fed into the engine is started. Accordingly, if the driving speed of the air flow control valve is suddenly increased or decreased (so-called skipping action) when the operation for increasing or decreasing the amount of secondary air is started, and thereafter slowly controlled, the equivalent air-fuel ratio will be maintained for a long time within the range wherein the three-way catalytic converter operates at its highest purifying efficiency. As a result, the effect of purifying pollutants will be further improved.